Preparation of biuret-containing polyisocyanates

ABSTRACT

A process for the preparation of polyisocyanates which contain one or more biuret groups, by reacting
     a) an aliphatic or cycloaliphatic isocyanate containing two or more isocyanate groups (isocyanate a) with   b) a tertiary alcohol or a mixture of water and a tertiary alcohol (biuretizing agent b)
 
at from 100 to 250° C., which comprises carrying out the reaction in the presence
   c) of a stabilizer (c) which constitutes a catalytic amount of urea, ammonia, biuret, a urea derivative of the formula I                  
   in which R 1 , R 2 , R 3  and R 4  are hydrogen, C 1  to C 10  alkyl or C 5  to C 10  aryl, or   a carboxamide of the formula II                  
   in which R 5  is C 1  to C 12  alkyl which is unsubstituted or in which 1, 2 or 3 hydrogen atoms are replaced by a radical

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the preparation ofpolyisocyanates which contain one or more biuret groups, by reacting

-   a) an aliphatic or cycloaliphatic polyisocyanate (isocyanate a) with-   b) a tertiary alcohol or a mixture of water and a tertiary alcohol    (biuretizing agent b)    at from 100 to 250° C.

2. Description of the Background

In the text below, the adjective “biuret-containing” indicates that thecompounds it describes have a content of biuret groups.

The preparation of biuret-containing polyisocyanates is a reaction whichhas been described at some length (cf. H. J. Laas et al., J. prakt.Chem. 336 (1994) 185–200).

Numerous patents disclose, for example, the reaction of water with anexcess of polyvalent isocyanates to give, first of all, urea groups,which undergo further reaction with the isocyanates to form biuretgroups (cf. DE-A 1 101 394). The difficulty of preparing homogenousmixtures of water and the isocyanate means that in the course of thisreaction, in practice, local excesses of water always result in theformation of greater or lesser proportions of insoluble polymericurea-containing compounds which are deposited in the reaction vessel orin the off-gas space.

U.S. Pat. No. 4,028,392 describes a process in which this problem isavoided by employing water in the form of an aqueous solution with asolvent which is inert to isocyanates. The disadvantage here is the needto separate the solvent from the product again by distillation.

SUMMARY OF THE INVENTION

These problems can be overcome using the process known from DE-A 1 543178, in which a monohydric tertiary alcohol such as tert-butanol is usedinstead of water. The alcohol reacts at 70° C. or more with an excess ofisocyanate to form biuret-containing polyisocyanates and, asby-products, an olefin—isobutene for example—and CO₂, which can beremoved from the reaction mixture with ease.

It is probable that the alcohol and the isocyanate react initially toform a urethane which decomposes into an amine, CO₂ and an olefin, andthat the amine reacts with further isocyanate to give urea derivatives,and then to give biuret-containing polyisocyanates.

This reaction is preferably carried out in the presence of catalysts,with those recommended for this being acids such as strong inoganicLewis and Brönstedt acids (cf. DE-A 1 543 178) and salts ofnitrogen-containing bases and inorganic and/or organic acids (cf. DE-A 1931 055).

Biuret-containing polyisocyanates are employed in particular in thepaint industry as curing agents in coating systems whose bindersgenerally comprise polymers having isocyanate-reactive groups.

So that the coating systems cure within a short period after applicationto a substrate to give coatings of good mechanical properties and highresistance to chemicals, it is necessary for the biuret-containingpolyisocyanates to have a high content of NCO groups and a high level ofreactivity with respect to the reactive groups in the binders.

In addition, the proportion of volatile isocyanates should be small evenafter prolonged storage, so as to enable safe processing of thebiuret-containing polyisocyanates without the need for special safetyprecautions. So that these can be used to produce coating systems whichexhibit good flow properties and a low solvent content, the paintindustry demands products which at the same time are of low viscosity.Furthermore, the inherent color of the products should be minimal.

The biuret-containing polyisocyanates prepared by the known processesfrom tertiary alcohols and isocyanates, however, leave much to bedesired, since they are too dark in color for many applications and, inparticular after prolonged storage, still include considerablequantities of readily volatile monomeric isocyanates.

It is the object of the invention to provide an economic process bywhose use it is possible to prepare biuret-containing polyisocyanateswhich are pale in color and whose content of volatile isocyanates, inparticular after prolonged storage, is low.

We have found that this object is achieved by a process for thepreparation of polyisocyanates which contain one or more biuret groups,by reacting

-   a) an aliphatic or cycloaliphatic isocyanate containing two or more    isocyanate groups (isocyanate a) with-   b) a tertiary alcohol or a mixture of water and a tertiary alcohol    (biuretizing agent b)    at from 100 to 250° C., which comprises carrying out the reaction in    the presence-   c) of a stabilizer (c) which constitutes a catalytic amount of urea,    ammonia, biuret, a urea derivative of the formula I

-    in which R¹, R², R³ and R⁴ are hydrogen, C₁ to C₁₀ alkyl or C₅ to    C₁₀ aryl, or-    a carboxamide of the formula II

-    in which R⁵ is C₁ to C₁₂ alkyl which is unsubstituted or in which    1, 2 or 3 hydrogen atoms are replaced by a radical

DETAILED DESCRIPTION OF THE INVENTION

Among the starting materials for the process of the invention, suitableisocyanates (a) are polyfunctional isocyanates, especially aliphatic andcycloaliphatic di- and triisocyanates containing 4 to 30 carbon atoms.Particular examples are diisocyanates X(NCO)₂ in which X is an aliphatichydrocarbon radical of 4 to 12 carbon atoms or a cycloaliphatichydrocarbon radical of 6 to 15 carbon atoms. Of particular significancein this respect are the commercially available starting compounds whichare prepared industrially by the phosgenization of diamines by theprocess as described, for example, in DE-C 20 05 309 and DE-A 2 404 773and by the phosgene-free process (biurethane cleavage) described inEP-B-0 126 299 (U.S. Pat. No. 4,596,678), EP-B-0 126 300 (U.S. Pat. No.4,596,679), EP-A-0 355 443 (U.S. Pat. No. 5,087,739) and EP-A-0 568 782.

These are, in particular, 1,6-diisocyanatohexane (HDI),1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (IPDI) andbis(4-isocyanatocyclohexyl)methane.

Starting compounds which are of less importance in practice but of equalsuitability in principle are isocyanates comprising 3 or more isocyanategroups, for example those which in addition include allophanate orisocyanurate groups. Examples of these are the corresponding derivativesof HDI which are prepared by trimerization of HDI (cf.Kunststoff-Handbuch, volume 7, pp. 94 to 96, 3rd edition, 1993, CarlHanser Verlag).

Particularly suitable biuretizing agents (b) are the tertiary alcoholsspecified in DE-A 1 543 178, ie. especially monohydric alcohols of 4 to20 carbon atoms, examples being 2-methyl-2-butanol, 2-methyl-2-pentanol,3-methyl-3-pentanol, 3-ethyl-3-pentanol, 3-ethyl-3-nonanol,3-methyl-1-butyn-3-ol, 3-methyl-1-pentyn-3-ol,3,5-dimethyl-1-hexyn-3-ol, 1-methylcyclopentanol, 1-methylcyclohexanol,1-ethylcyclohexanol, 1,1-diphenylethanol, 1,1,2-triphenylethanol and, inparticular, tert-butyl alcohol. Mixtures of these alcohols are of coursealso suitable.

In addition to the tertiary alcohols, water in the form of an aqueoussolution with the tertiary alcohols can also be used to biuretize theisocyanates (a). In this context, particularly suitable solutions oftertiary alcohol and water are those containing up to 80 mol %,preferably up to 40 mol %, of water, based on the sum of the componentsof the mixture, since at these mixing ratios water is incorporatedhomogeneously and no oligomeric or polymeric urea derivatives, whichprecipitate from the reaction mixture, are formed in the course of thereaction with the isocyanates (a).

In accordance with the invention, the isocyanate (a) is reacted with thebiuretizing agent (b) in the presence of catalytic amounts of astabilizer (c).

Suitable stabilizers (c) are urea, ammonia, biuret, a urea derivative ofthe formula I

in which R¹, R², R³ and R⁴ are hydrogen, C₁ to C₁₀ alkyl, preferablymethyl or ethyl, or C₅ to C₁₀ aryl, preferably phenyl or benzyl, ora carboxamide of the formula II

in which R⁵ is a C₁ to C₁₂ alkyl, preferably C₁ to C₆ alkyl, which isunsubstituted or in which 1, 2 or 3 hydrogen atoms are replaced by aradical

Examples of suitable urea derivatives are N-methylurea,N,N-dimethylurea, N,N′-dimethylurea, N-ethylurea, N,N-diethylurea,N,N′-diethylurea, ethyleneurea and N-phenylurea.

Suitable carboxamides of the formula II are formamide,N-methylformamide, acetamide, malonamide and succinamide.

The stabilizers (c) are preferably employed in quantities of from 0.01to 2.0 mol %, and with particular preference in quantities of from 0.05to 1 mol %, based on the isocyanate groups in (a).

Using the process of the invention, the biuret-containing polyisocyanatecan be prepared either continuously or batchwise.

A suitable apparatus for continuous preparation is, for example, areactor cascade comprising a plurality of individual reactors throughwhich there is a continuous flow.

Batchwise preparation can be carried out, for example, in a stirredreactor.

Normally, the isocyanate (a) is taken as initial charge and thebiuretizing agent (b), in which the stabilizer (c) is advantageouslyalready dissolved, is metered in.

The reaction is preferably carried out in bulk, although to reduce theviscosity it is also possible to use a solvent which is inert toisocyanate groups. Suitable solvents are those mentioned in DE-A 1 543178, dioxane, tetrahydrofuran, triethylene glycol diacetate, toluene,benzene, chlorobenzene, o-dichlorobenzene, butyl acetate, ethyleneglycol monoethyl ether acetate and methylene chloride.

In general the reaction is carried out under atmospheric pressure,although higher pressures of 1 to 10 bar are advisable, for example,when using solvents or isocyanates (a) which boil below the preferredreaction temperatures.

At the preferred temperatures, the reaction times are in general from 2to 5 h. The reaction time is advantageously chosen such that thetheoretical NCO value is reached at the end. The theoretical NCO valueis that NCO value possessed by the reaction mixture if the entirequantity of biuretizing agent employed has formed the quantity of biuretgroups which are to be expected from theory.

As is known, the result of reacting an isocyanate group with a moleculeof water or tertiary alcohol is an amino group which reacts with twofurther isocyanate groups to form a biuret group. Since the startingcompounds employed include polyfunctional isocyanates, the growth of thebiuret-containing polyisocyanates therefore takes place in accordancewith the kinetics of crosslinking reactions (cf. B. Vollmert, Grundriβder Makromolekularen Chemie, volume II, pp. 247 to 260, Vollmert-Verlag,Karlsruhe, 1988), with each biuret group forming a branching point. Inorder to avoid the formation of relatively large branched-chainassociations with two or more branching points, or even the occurrenceof gelling, it is generally advisable to employ from 0.5 to 20 mol %,preferably from 2 to 10 mol %, of biuretizing agent, based on theisocyanate groups in (a).

Under these conditions, the isocyanates (a) react with the biuretizingagents predominantly to form mixtures of biuret-containingpolyisocyanates whose principal component comprises thosebiuret-containing polyisocyanates which are composed of three unitsderived from the isocyanate (a), containing only one biuret group.

Otherwise, it is possible by simple prior experimentation or calculationto determine the stoichiometric ratios at which mixtures ofbiuret-containing polyisocyanates are formed which have the desiredaverage degree of polymerization.

In general, in order to obtain products which do not release hazardousquantities of isocyanates during processing, it is necessary to separateoff the majority of the unreacted isocyanates (a) from thebiuret-containing polyisocyanates formed. The usual desire is forproducts whose content of monomeric isocyanates (a) is less than 1% byweight, preferably less than 0.5% by weight, based on saidbiuret-containing polyisocyanates. The separation of the isocyanates (a)is advantageously carried out under reduced pressure at between 50° C.and the chosen reaction temperature, for example by distilling off theseisocyanates.

In the paint industry, the desire is in particular for biuret-containingpolyisocyanates wich are substantially free of solvents and from theisocyanates (a) used as starting materials, and which have a viscosityof from 2000 to 15,000 mPa·s, preferably from 2500 to 10,000 mPa·s(measured at a temperature of 23° C. and a shear gradient of 100 s⁻¹).

Products with these viscosities are in general obtained when thestoichiometry of the starting products, the isocyanates (a) and thebiuretizing agents (b), is chosen in accordance with the recommendation.

The products obtained by this process are distinguished in particular inthat they couple comparatively low viscosity and a low content ofvolatile isocyanates of low molecular weight, like the isocyanates (a)used as starting materials, with a high NCO content and a highreactivity with respect to the binders employed in coatings, saidbinders containing isocyanate-reactive groups and being, for example,hydroxyl-containing polyacrylates. Particular advantages are that thecontent of volatile isocyanates does not rise even on prolonged storageof the products, and that the products are substantially colorless.

The products obtained by the process of the invention are particularlysuitable as curing agents in the paint industry. The processing of thesecuring agents to give coating formulations, and the coatings producedtherefrom, are items of general knowledge.

EXAMPLES General Preparation Procedure for the Biuret-containingPolyisocyanates (a)

504 g (3 mol) of 1,6-hexamethylene diisocyanate (HDI) are charged undernitrogen blanketing to a 1 stirred reactor, and are heated to thereaction temperature indicated in the tables below. Then 14 mol %, basedon the HDI, of biuretizing agent (b) and, dissolved therein, 0.2 mol %,based on the HDI, of the stabilizer (c) or of the acidic catalyst areadded over the course of 2 min and the reaction mixture is stirred for 3h. The reaction mixture is then distilled on a thin-film evaporator at165° C. and 2.5 mbar.

Departing from the above indications, the quantity of urea employed was

-   0.4 mol % in Example 11,-   0.6 mol % in Example 12, and-   1.0 mol % in Example 13,    based in each case on the quantity of HDI.

TABLE 1 NCO Monomer content Biuretizing agent Stabilizer Temp. contentViscosity C N 0 d 21 d Ex. (b) (c) [° C.] [% by wt.] [mPa · s] [Hazen][% by wt.] [% by wt.] 1 tert-Butanol (tBuOH) UR 180 22.0 4350 5 0.150.25 2 tBuOH Eth UR 180 22.7 2290 10 0.20 0.41 3 tBuOH:water 19:1 UR 17022.4 3340 7 0.08 0.22 4 tBuOH:water 19:1 UR 190 22.0 6030 10 0.10 0.21 5tBuOH:water 19:1 Eth UR 180 22.7 2200 12 0.15 0.45 6 tBuOH:water 19:1 DMUR 180 22.7 2280 15 0.13 0.43 7 tBuOH:water 4.6:1 UR 180 22.2 5550 50.11 0.23 8 tBuOH:water 1.8:1 UR 180 22.0 6480 2 0.13 0.28 9 tBuOH:water1:1 UR 180 22.2 5450 4 0.14 0.31 10 tBuOH:water 0.27:1 UR 180 21.412,600 10 0.14 0.28 11 tBuOH:water 1:1 UR 180 22.0 6120 12 0.12 0.27 12tBuOH:water 1:1 UR 180 21.3 11,560 18 0.12 0.29 13 tBuOH:water 1:1 UR180 20.8 18,200 22 0.13 0.25 14 tBuOH:water 19:1 Biuret 180 22.0 3860 150.14 0.27 15 tBuOH:water 19:1 Acetamide 180 22.6 3020 0.17 0.31 16tBuOH:water 19:1 Samid 180 22.5 3000 0.14 0.34 17 tBuOH:water 19:1Ammonia 180 22.0 2340 28 0.21 0.50

TABLE 2 NCO Monomer content Comp. Biuretizing agent Acidic Temp. contentViscosity C N 0 d 21 d Ex. (b) catalysts [° C.] [% by wt.] [mPa · s][Hazen] [% by wt.] [% by wt.] 1 tert-Butanol (tBuOH) BF₃ 150 22.9 2550206 0.09 0.69 2 tBuOH PTSS 150 21.7 5400 350 0.05 0.48 3 tBuOH DEHP 18022.0 4840 42 0.07 0.42 4 tBuOH EHA 180 22.0 4660 38 0.09 0.42 5 tBuOHHAc 180 22.1 4330 55 0.08 0.40 6 tBuOH — 180 22.9 2130 44 0.09 0.53 7tBuOH:water 19:1 PTSS 180 22.0 5550 371 0.11 0.91 8 tBuOH:water 19:1PTSS 150 21.8 5360 256 0.03 0.49 9 tBuOH:water 19:1 DEHP 180 22.4 380032 0.10 0.53 10  tBuOH:water 19:1 EHA 180 22.4 3650 10 0.15 0.63 11 tBuOH:water 19:1 ClAc 180 22.3 3970 56 0.14 0.53 12  tBuOH:water 19:1 —180 22.7 2090 32 0.12 0.61Notes on Tables 1 and 2Compounds Employed

The biuretizing agents employed were tert-butanol (tBuOH) and mixturesthereof with water. The figures given thereafter indicate the molarratio of the components in the mixture

-   UR=urea-   EthUR=ethyleneurea-   DM UR=N,N′-dimethylurea-   BF₃=boron trifluoride as the dihydrate-   PTSA=p-toluenesulfonic acid-   DEHP=di(2-ethylhexyl) phosphate-   EHA=2-ethylhexanoic acid-   HAc=acetic acid-   Samid=succinamide-   ClAc=chloroacetic acid-   Ammonia=ammonia in the form of a 25% strength by weight aqueous    solution    NCO Content:

The NCO content is given in % by weight and was measured in accordancewith DIN 53 185.

Viscosity:

The viscosity data relate to measurements made at 23° C. with a sheargradient of 100 s⁻¹.

Color Number (CN):

The color number was determined in accordance with DIN ISO 6271 and isindicated in Hazen scale units.

Monomer Content:

The monomer content indicates the quantity of monomeric isocyanate in %by weight present in the respective biuret-containing polyisocyanatedirectly after preparation (0 d) or after storage for 21 days at 50° C.(21 d). It was measured in accordance with DIN 55 956.

1. A process for the preparation of a polyisocyanate which contains oneor more biuret groups by reacting a) an aliphatic or cycloaliphaticisocyanate containing two or more isocyanate groups (isocyanate (a))with b) 0.5 to 20 mol % based on the isocyanate groups in (a) of atertiary alcohol or a mixture of water and a tertiary alcohol(biuretizing agent (b)) at from 100 to 250° C., which comprises carryingout the reaction in the presence c) from 0.01 to 2.0 mol % based on theisocyanate groups in (a) of a stabilizer (c) selected from the groupconsisting of urea, ammonia, biuret, ethylene urea, a urea derivative ofthe formula I

in which R¹, R², R³ and R⁴ are hydrogen, C₁ to C₁₀ alkyl or C₆ to C₁₀aryl, or a carboxamide of the formula II

in which R⁵ is C₁ to C₁₂ alkyl which is unsubstituted or in which 1, 2or 3 hydrogen atoms are replaced by a radical


2. A process as claimed in claim 1, wherein the isocyanate (a) is a C₄to C₃₀ diisocyanate or triisocyanate.
 3. A process as claimed in claim1, wherein the isocyanate (a) is hexamethylene-1,6-diisocyanate.
 4. Aprocess as claimed in claim 1, wherein the biuretizing agent (b) is atertiary alcohol or a mixture of a tertiary alcohol and up to 80 mol %of water based on the sum of the components of the mixture.
 5. A processas claimed in claim 1, wherein the tertiary alcohol is tert-butanol. 6.A process as claimed in claim 1, wherein the reaction is carried out atfrom 140 to 220° C.
 7. A process as claimed in claim 1, wherein thepolyisocyanate containing biuret groups is prepared and then unreactedisocyanate (a) is removed from it down to a content of less than 0.5% byweight, based on the polyisocyanate which contains biuret groups.
 8. Aprocess as claimed in claim 1, wherein the stabilizer (c) is urea.
 9. Aprocess as claimed in claim 1, wherein the stabilizer (c) is ammonia.10. A process as claimed in claim 1, wherein the stabilizer (c) isbiuret.
 11. A process as claimed in claim 1, wherein the stabilizer (c)is ethyleneurea.
 12. A process as claimed in claim 1, wherein thestabilizer (c) is a urea derivative of the formula I.
 13. A process asclaimed in claim 12, wherein the stabilizer (c) is N,N′-dimethylurea.14. A process as claimed in claim 1, wherein the stabilizer (c) is acarboxamide of the formula II.
 15. A process as claimed in claim 14,wherein the stabilizer (c) is acetamide.
 16. A process as claimed inclaim 14, wherein the stabilizer (c) is succinamide.